The invention relates to rigid endoscopes for viewing into generally inaccessible areas, such as various body cavities or industrial components. Endoscopes for remotely viewing into body cavities or industrial components are well-known. Such endoscopes can be divided into two categories, rigid and flexible. Most flexible endoscopes utilize fiber optic image transfer bundles to carry the image from the distal tip to the proximal eyepiece. Other flexible endoscopes employ a small CCD chip at the distal tip and use fiber optics only for illumination and not for direct transfer of the image.
Most modern rigid endoscopes are of the "rod-lens" type, which utilize a train of rod-like lenses to relay the image of an object from a distal end to a proximal eyepiece or other viewing means, such as a CCD camera. Such a device is taught in U.S. Pat. No. 4,168,882 to Hopkins, whose entire disclosure is incorporated herein by reference. In that device, the image of an object area is formed by a first distal lens. A second lens reimages the first image to a third lens, which reimages the second image to a fourth lens, and so on, until the image is relayed to the viewing means.
Rigid endoscopes employing a train of lenses for image transfer require many lenses in order to transfer the image through a slender tube while retaining good resolving capabilities and a wide field of view. It is necessary that endoscope probe lengths reach as high as 30 -40 centimeters, and such lengths require as many as 20 -30 lenses in order to transmit a clear, bright image. Each lens requires grinding, polishing, and coating to obtain high optical quality, and must be aligned and mounted in a rigid encasement with great precision. As a result, the cost of producing such rigid endoscopes is very high. Further, such expensive rod-lens construction precludes the feasibility of removal and disposal of the whole shaft after a single use, which would be a preferred option, especially for cleanliness in medical applications. Removable, disposable sheaths which cover the length of the shaft have been proposed. However, such sheaths do not provide adequate protection against bacterial contamination, are cumbersome to work with, and are susceptible to breakage.
Stereo versions of rod-lens type endoscopes are also known in the prior art. For example, U.S. Pat. No. 5,122,650 to McKinley, the entire disclosure of which is incorporated herein by reference, teaches such an endoscope. However, substantial modifications to the basic rod-lens design must be made to obtain independent optical paths in the shaft portion in order to yield two images containing parallax data, such images being essential for production of a stereo image. These modifications require much duplication of optical elements, especially in the shaft portion, which results in an endoscope which is costly and is even less suitable for disposability of the shaft.